Generally, pipes are roughly classified into welding pipes and seamless pipes. An electric resistance welding pipe as one of the welding pipes is manufactured by rounding a sheet by roll forming or the like, and then confronting each edge and welding. In the welding pipes, toughness and strength of weld are generally bad compared with those of a mother strip. In use of the pipes, it is an inevitable problem to ensure certain toughness and strength of weld for each application.
For example, since a line pipe for transporting crude oil or natural gas is often laid in a cold latitude, low temperature toughness is essentially concerned. Moreover, strength is importantly concerned with a casing for protecting a mining pipe in an oil well for mining crude oil.
Typically, a hot-rolled sheet (strip) to be a mother strip of an electric resistance welding pipe is subjected to composition design or heat treatment in consideration of properties of the mother strip after being formed into a pipe so that properties of the mother strip such as toughness and strength are secured.
However, since characterization of welded seams is severely affected by a method of the electric resistance welding rather than by the composition design or heat treatment of the mother strip, welding technique needs to be essentially improved.
In many cases, the following phenomenon may by a cause of problems for electric resistance welding, that is, oxides called penetrators are generated on a strip edge, and the penetrators are not discharged from the edge along with discharge of melting steel during electric resistance welding and remained thereon, and the remained penetrators cause reduction in toughness, leading to insufficient strength.
Thus, to remove the penetrators which are causes of conventional electric resistance welding failure from weld, earnest investigation has been made so far on an active discharging technique of melting steel from a strip edge in the weld. For example, JP-A-57-031485, JP-A-63-317212, JP-A-2001-170779 and JP-A-2003-164909 describe examples of investigation on a shape of each strip edge.
Moreover, JP-A-2001-259733 describes that lateral edges of a strip are chamfered in various shapes in order to facilitate adjustment of confronting pressure on edges at both sides of a strip in welding of an electric resistance welding pipe, and to improve welding reliability.
In JP-A-57-031485, JP-A-63-317212, JP-A-2001-170779 and JP-A-2003-164909, it is intended that each strip edge is shaped with tapering so as to discharge the penetrators along with discharge of melting steel. The reason for shaping the tapering is considered to be due to a fact that the strip edge can be easily shaped with the tapering. However, when the strip edge is shaped with straight or planar tapering having a uniform slope, defects such as oxides may have not been adequately discharged along with discharge of melting steel.
Moreover, JP-A-2001-259733 discloses various chamfer shapes for facilitating adjustment of confronting pressure. However, no description is found therein on a point of discharge of penetrators along with discharge of melting steel, and a point of improvement in characterization of welded seams (particularly low temperature toughness) through such discharge of penetrators. Therefore, a particular shape that is effective for improving the characterization of welded seams (particularly low temperature toughness) is completely unknown among the various chamfer shapes disclosed therein.
In an actual manufacturing process of electric resistance welding pipes, strips having various kinds of thickness are subjected to roll forming to be into pipes. When means for shaping the tapering on each strip edge is solely used, for example, a cutting bite or a shaving roll needs to be adjusted in position to be installed at an accurate interval similar to strip thickness, which has been a cause of significant reduction in production efficiency.
Thus, it could be advantageous to provide a method of efficiently manufacturing electric resistance welding pipes having excellent characterization of welded seams, which may adequately accelerate discharge of defects such as oxides along with discharge of melting steel, in addition, may not cause reduction in production efficiency.